Investors had a strong appetite for the company's stock, chomping up shares priced at $26 for the debut, and pushing the stock price up more than 50 percent soon after the opening bell at the New York Stock Exchange. Shares settled at about $36 by mid-day. The first-day pop pushed GrubHub's value to more than $3 billion.

GrubHub said it had hoped to raise $100 million in its offering, but instead raised close to twice that amount. That's not bad for a startup whose owners, Matthew Maloney and Michael Evans, walked the Chicago streets beating the bushes for business back in 2004.

"I bought a Sales For Dummies book and I went door to door, signing up restaurants," Evans told Inc. earlier this year.

Today, GrubHub's service is available in 600 cities, including London, and 30,000 restaurants. It counts 34 million active users and 135,000 orders per day. GrubHub targets small, independent restaurants in the U.S., or about 61 percent of the market, it reports. That market is worth about $67 billion in sales, the company says.

Like Twitter and dozens of other companies that have gone public in the last couple of years, GrubHub took advantage of a provision in the JOBS Act of 2012, which lets small companies valued at $1 billion or less to file for an initial public offering in private, and just a few weeks before it intends to debut. That provision allows smaller companies to avoid an extended period of public scrutiny, which could potentially damage sales or reputation.

According to GrubHub's S-1 filing--the form a company must submit to the Securities and Exchange Commission prior to going public--profits have declined over the past three years as the company has expanded. For the full-year 2013, it reported $6.7 million in net income on $137 million in revenue, a 55 percent decrease from 2011, when GrubHub reported $14.8 million in profits on $60 million of sales.

Expenses for sales and marketing have more than doubled to $37 million over the same two-year period, as have costs for operations and support, and investment in technology, which GrubHub reported cost $34 million and $15 million respectively.

Among the risks that GrubHub lists in its filing papers are practical issues related to the merger with Seamless, and the ability of the restaurants it works with to maintain quality and service expectations of customers. Other questions for the company include: whether the strength of the economy and its impact on consumer discretionary spending might prove a negative force going forward, as well as the financials of smaller restaurants that make up GrubHub's network.

"Many of the factors affecting restaurant costs are beyond the control of the restaurants in our network," GrubHub said in its filing papers. "In many cases, these restaurants may not be able to pass along these increased costs to diners and, as a result, may cease operations, which could harm our profitability and results of operations."